1. Field of the Invention
This invention relates to an amplifier appropriate for an output stage, etc., of an audio amplifier.
2. Description of the Related Art
FIG. 1 shows an example of a conventional power amplifier. Shown in FIG. 1 is a generally used OCL (output capacitorless) complementary SEPP (symmetrical single-ended push-pull) circuit, wherein IN is an input terminal to which an input signal from the preceding stage is applied and A is an amplifier. One input terminal + of the amplifier A is connected to the input terminal IN and is also grounded via a resistor Rg; the other terminal - is grounded via a resistor Re.
+B and -B are power supplies of opposite polarity to each other and Q1 and Q2 are output transistors of opposite polarity to each other. Q1 is an npn transistor and Q2 is a pnp transistor. The power supplies +B and -B are connected to the collectors of the transistors Q1 and Q2 so that they become forward bias polarities with respect to the transistors Q1 and Q2 respectively. That is, the positive electrode of the power supply +B is connected to the collector of the transistor Q1 and the negative electrode of the power supply -B to the collector of the transistor Q2. OUT is an output terminal and RL is a load connected to the output terminal OUT.
A complementary symmetrical circuit consisting of the transistors Q1 and Q2 has bases connected in common to provide a signal input terminal, which is connected to an output terminal of the amplifier A, and emitters connected in common to provide a signal output terminal, which is connected via a resistor Rf to the input terminal of the amplifier A and is also connected to the output terminal OUT. In the circuit, the transistor Q1 or Q2 operates as an emitter follower in response to the positive or negative polarity of an input.
An input signal from the preceding stage is applied to the input terminal IN and is amplified by the amplifier A and further the output of the amplifier A is amplified by the output transistors Q1, Q2 at the following stage, then the result is supplied to the load RL. That is, the current flowing into the output transistors flows into the load RL.
However, at such a power amplifier, a value resulting from adding a voltage loss at transistors, etc., to the maximum output voltage is required as a supply voltage. The supply voltage minus the output voltage is applied between the collector and emitter of the output transistor and power resulting from multiplying the voltage value by output current becomes a power loss at the output transistor. Collector-emitter voltage V.sub.CE at the maximum output differs from that at the normal operation. When the collector-emitter voltage is large, a power loss at the output transistor becomes great, heating the output transistor. Thus, expensive transistors or cooling means for coping with the heating is required. Also, the collector-emitter withstand voltage of the output transistor needs to be at least twice as high as the supply voltage.
Then, an amplifier has been proposed where a formerly fixed supply voltage from a power supply is made variable and a supply voltage responsive to an input signal level is supplied, thereby reducing a collector loss. FIG. 2 shows such a power amplifier having positive and negative power supplies. A first SW (switching) power supply 4 and a second SW power supply 5 are connected to collectors of a first transistor 2 and a second transistor 3 to which output signals of a power amplifier 1 are applied. An output signal generated at a terminal 7 of a load 6 is applied via a direct current power supply for level shift, 8 or 9, to a first or second control circuit 10 or 11.
Now assume that an output signal indicated by a dotted line in FIG. 3 is generated at the load 6. During the period of a positive half cycle of the output signal, the first control circuit 10 turns on/off a switch 12 so as to generate a supply voltage V.sub.BA of size responsive to the level of the output signal as denoted by a solid line in FIG. 3. The switching ratio of the switch 12, i.e. the duty cycle of the output signal from the switch 12 is adjusted to determine the voltage of a capacitor C. A ripple filter is formed by the capacitor C and a coil L. A diode D is provided to set the end of the coil L on the side of the switch 12 to a ground potential when the switch 12 is turned off. The second control circuit 11 holds the output supply voltage V.sub.BB value small at a given level in response to the fact that the output signal is positive. In contrast, when the output signal generated at the load 6 enters a negative half cycle, the first control circuit 10 holds the supply voltage V.sub.BA constant and the second control circuit 11 changes the supply voltage V.sub.BB in response to the output signal level. As a result, the supply voltages V.sub.BA and V.sub.BB change as denoted by solid lines in FIG. 3. As seen from FIG. 3, the supply voltages change following the level of the output signal indicated by the dotted line, thus the collector-emitter voltages V.sub.CE of the first and second transistors 2 and 3 are always maintained at constant values Vx (values of the DC power supplies 8 and 9), and a collector loss can be suppressed.
By the way, for light weight, a configuration of a single power supply with no output transformer is required for a power amplifier for car audio, etc. Thus, a BTL (balanced transformerless) drive circuit is generally used. FIG. 4 shows a power amplifier of a single power supply using SW power supplies with a BTL drive. The power amplifier shown in FIG. 4 basically has two units in FIG. 2 at the left and right. A second power amplifier 13 inverts an input signal and applies the resultant signal to a load 6. Thus, AC signals of opposite phase to each other are applied to both ends of the load 6, as shown in FIGS. 5 and 6. The signal indicated by a dotted line in FIG. 5 is generated on the terminal 7 side of the load 6 and the signal indicated by a dotted line in FIG. 6 is generated on the terminal 14 side of the load 6. Supply voltages V.sub.BA and V.sub.BB generated at collectors of first and second transistors 2 and 3 are indicated by solid lines in FIG. 5. Supply voltages V.sub.BC and V.sub.BD generated at collectors of third and fourth transistors 15 and 16 are indicated by solid lines in FIG. 6.
Therefore, power amplification of the BTL driver with less power consumption can be performed according to the circuit shown in FIG. 4.
In the examples given above, the load RL 6 is a loudspeaker, into which a current flows to generate sound.
However, the power amplifier in FIG. 4 requires four SW power supplies. Choke coils required at the SW power supplies are expensive and become noise sources because they generate a strong magnetic field during the switching operation. Thus, reduction of the number of SW power supplies has been desired.